1. Field of the Invention
The present invention relates to the field of semiconductor fabrication and testing equipment and, more particularly, to an improved method and apparatus for mounting and adjusting probe card needles used to test fabricated semiconductor wafers.
2. Background Art
One step in the process of semiconductor manufacturing is the testing of circuits fabricated on wafers. During testing, a wafer of monolithic circuits is placed in a holder under a microscope and is tested using a multiple-point probe card. Metal probes are used to contact various bonding pads on an individual circuit, and a series of electrical tests are performed to determine the electrical properties of the selected circuit. Information from these tests is compared with information stored in a memory, and the circuit is either rejected or accepted based on this comparison.
FIG. 1 illustrates a "probe card" probe assembly commonly used to electrically test wafer devices. A probe card is a printed circuit board that can be used in conjunction with standardized testing equipment to test the electrical properties of semiconductor chip devices. A typical probe card contains a plurality of probe assemblies 100-107, matching circuitry, and an interface for attaching the probe card to the testing equipment. Each probe assembly contains a probe needle. During the testing procedure, the probe needle is brought into contact with aluminum bonding pads located on the individual semiconductor circuit 110 so that sufficient electrical contact is established to allow the electrical tests to be performed.
To assure proper contact with the bonding pads, the probe needle tips must penetrate the thin aluminum oxide layer that characteristically forms over the exposed aluminum bonding pads. To do so, probe needles are typically oriented at an angle and are applied to the surface of the wafer with sufficient force so that when the bonding pads of the wafer are brought into contact with a probe needle tip, the probe needle scratches, or "scrubs," the surface of the bonding pad, penetrating any built-up aluminum oxide layer. In addition, all probe needles that contact the bonding pads should be co-planar. That is, the probe needle tips should all lie in a single plane.
A typical probe assembly is shown in FIG. 2. The probe assembly of FIG. 2 consists of a thin metal blade 201 and a specialized probe needle 202. The blade 201 is connected to the matching circuitry and interface 203. The probe needle 202 is attached to the blade 201 at a shoulder 204 at an angle of approximately six degrees below horizontal. The probe needle is typically round in cross section. The probe needle 202 is of uniform diameter from the shoulder 204 to approximately the midsection of the needle. The probe needle 202 is tapered from the midsection to the end of the probe needle tip 205. The tip is typically 7 mils long and is bent at an angle approximately sixteen degrees from vertical. The tip 205 is a part of the needle assembly and is formed by the bend.
As each silicon wafer is tested, the probe assemblies are lowered so that the tips of the probe needles make contact with the semiconductor circuit's aluminum bonding pads. The combination of an untapered needle diameter and the length of the tapered portion of each probe needle gives the probe needle a predetermined amount of flexibility. Each probe needle is designed so that as the probe needle is lowered onto the pad, the probe needle tip "scrubs," or scratches, the surface of the aluminum bonding pad. Scratching is necessary to insure good electrical contact through the surface layer of accumulated aluminum oxide. The probe needle is attached to the blade and the tip is angled from the needle at angles designed to produce the optimum scratch mark on the bonding pad. The angles noted above, six and sixteen degrees, are given by way of example only. Other angles may be used.
When a probe card is first assembled, the probe card technicians must mount each probe needle assembly so that the tip of each probe needle, when lowered, will come in contact with the associated bonding pad on the wafer. Ideally, the probe card needle tip should initially contact the bonding pad at a point approximately 1/3 of the way across the pad, scrub through the center of the pad, and stop at a point approximately 2/3 of the way across the bonding pad. Mounting the probe needle in this manner insures that the needle tip scratches through the oxide layer and makes good electrical contact with the aluminum bonding pad. Unfortunately, it is difficult for the technician to estimate, without the benefit of any reference, the location of a point 1/3 of the way across the bonding pad. In practice, therefore, the technicians are directed to estimate the bonding pad center, and place the needle/blade, or mount the needle in the epoxy ring, so that the probe needle tip will approximately hit the bonding pad center.
FIG. 10 illustrates the different scrub marks created by starting the probe needle tip 1/3 and 1/2 of the way across the bonding pad 300. In typical probe needle mounting, the needle tip 1001 first contacts the bonding pad 300 in the center, and comes to rest near the edge of the bonding pad box. Because of the needle tip's proximity to the bonding pad edge 1006, "pushed" metal 1002 caused by the scrub may be forced out of the bonding pad area. In ideal probe card needle mounting, the needle tip 1003 first contacts the bonding pad 300 at a point 1/3 of the way across the bonding pad, and comes to rest at a point 2/3 of the way across the bonding pad. A needle mounted in this manner scrubs through the center of the pad, insuring good electrical contact, and comes to rest far enough away from the bonding pad edge 1006 so that the pushed metal 1004 seldom leaves the bonding pad region. Unfortunately, because of the technicians' inability to accurately and consistently estimate a point 1/3 of the way across a bonding pad, the current practice is to mount the probe needles in the center of the bonding pad, as shown by probe needle 1001.
After prolonged use, the probe needle tips of the probe assemblies typically lose co-planarity. Consequently, inaccuracies may occur in testing the circuits because either a higher probe needle tip fails to make sufficient contact to scratch through the layer of aluminum oxide, or a lower probe needle tip scratches the bonding pad excessively, damaging the circuit, or the probe needle tip, or both.
To prevent these testing inaccuracies from occurring, probe card technicians monitor the wafer testing procedure and ensure that the probe needle tips remain co-planar. In the prior art, technicians select a representative wafer at random from the group of wafers that have already been tested with the probe card. The technicians re-planarize the probe card needle tips at a planarization station. One type of planarization station is called a light box. The probe card is lowered onto chucks that are grounded to a set of light emitting diodes (LED). Each time a probe needle touches a chuck an LED lights up. The amount of travel between the point where the first LED illuminates and the last LED illuminates represents the planarity error. The technicians adjust the angle and position of each probe needle until the planarity error is within a specified range.
Next, using a low magnification microscope, the technicians inspect the surface of the wafer's bonding pads. By looking at the length and character of the scrub marks on each bonding pad, the technician performs a visual probe mark "footprint analysis" without the benefit of any specific visual reference. The technician typically is directed to look for actual presence and general location of the probe needle scrub marks. For example, a technician may be directed to make sure that less than 50% of each scrub mark is outside the bonding pad without cutting neighboring metal runs more than 50%. The technicians also look for overdrive damage (indicated by passivation exposure), and side to side scrub patterns that may indicate lanarity problems. Of course, the relatively small size of the bonding pad (approximately 4.times.4 mils) increases the difficulty of accurate footprint analysis.
One disadvantage of the prior art needle mounting and scrub mark inspection technique is that it is a subjective probe mark analysis that does not set any specific minimum or maximum limit to the length or difference in the acceptable scrub marks. A very short "dot" scrub mark indicates the absence of any oxide breaking scrub action, possibly preventing the probe needle from making good electrical contact with the bonding pad, and leading to the unwitting failure of a good wafer during testing. A long probe mark indicates excessive chuck travel, loss of probe needle tip planarity, or improper needle angle. In current practice, only scrub mark presence and location (but not length) are considered. Planarity checks are typically performed to determine accurate needle height placement. However, these planarity checks do not take into account actual needle length, loose probes, different needle gram force ratings, different taper lengths, defective probe card pads, epoxy plasticity, needle tip shape or diameter, or false readings due to needle contact anomalies, improper set-up, or equipment manufacturers' tolerance drift. The typical result is a probe card that, while being perfectly planarized, has widely varying scrub marks.
Further, as the probe needle is driven past initial wafer contact, it moves towards the center of the needle pattern. This over-travel (typically 3 mils) pushes the needle from pad center towards, and sometimes beyond, the inside edge of the bonding pad. This may result in pushed aluminum shorts, metal migration problems over time, loss of probe contact, burn-in failures, loose aluminum flakes inside packages, reductions in reliability, cut metal runs, and optical inspection rejects. To complicate the situation, as each probe needle wears, it also recedes towards the outside of the bonding pad box. This is partially offset by probe needle stress relaxation which decreases the average needle tip to probe card distance thereby effectively moving the needle tip array in towards the center of the needle pattern. At present, no current method allows and operator to place a probe needle in the ideal location (outer 1/3 line of the bonding pad box), resulting in improperly and/or inconsistently placed needles. Although epoxy cards allow accurate probe placement during assembly, these cards are typically mounted in the center of the bonding pad.
Prior art tools are currently available to adjust, or "tweak," the probe needles. These tools allow the technicians to push, pull, or twist the needles in any location or manner deemed best by the technician. The thinner midsection and tapered portions of the probe needle are more flexible than the thicker, untapered portion. Using these prior art tools, technicians typically adjust the probe needles where it is easiest for them to do so, that is, near the probe needle tip, where the probe needle is thinnest and easiest to bend. Due to the currently available tool inadequacies, some technicians even adjust the probe needles using home-made tools made from tweezers or needles.
Ideally, any adjustments to the probe needle should be made at the non-tapered portion of the needle or "shoulder" 204, where the probe needle 202 attaches to the probe card blade 201. Adjustments made at this location will have a minimum effect on the probe needle's flexibility and angle of attack. However, this type of bend in the thicker portion of the probe needle is very difficult to attain using any of the currently available tools.
The disadvantages of the prior art adjustment tools are multiple. Flexibility of the probe needle is provided by its tapered end portion. Consequently, any adjustments made to this critical area causes an intensified change in the probe needle's rated modulus of elasticity. The modulus of elasticity determines the amount of force to be applied by the probe needle tip, for a given needle diameter at a given overtravel setting. The preservation of this flexibility is critical in producing and maintaining a proper scratch mark on the bonding pad. Additionally, adjustments along the probe needle's tapered portion have a multiplied effect on the needle's angle of attack and scrub rate. When the technicians use the prior art tools to pull or push the probe needle along the tapered portion, distributed arc bends are typically applied to the probe needle. Arc bends have an intensified effect on the probe needle's modulus of elasticity, and also reduce the probe needle's ability to properly maintain or hold adjustments.
The prior art mounting, inspection, and adjusting techniques lead to inconsistent maintenance results and, thus, more frequent needle adjustment cycles and more down time in the wafer testing procedure. Inaccurate probe needle adjustments decrease wafer yield due to insufficient or excessive scrubbing between the probe needle tip and each circuit's bonding pads. In addition to this risk of reduced electrical test yield, the wafers may be rejected for visual damage. Costly rework is an option if the damage is less than specification limits. However, some military specifications prohibit rework by disallowing a second set of probe works on certain devices. Wire bonding integrity is tested by pull testing that is dependent on the presence of aluminum (without passivation exposure) in the "center" of the die. Unfortunately, this area is typically exposed by low or overdriven probes. Apparent yield improvement typically encourages the technicians to improperly perform overdrive increases made to "chase" a high or light probe needles. However, visual rejects and poor wire bond integrity result even if only one probe is overdriven.